Bug 17209: Remove use of onclick from masthead

Bug 17209: Remove use of onclick from masthead

There are two instances which use onclick in the OPAC masthead: clearing
search history, and logging out.

To test:
Confirm that clearing search history using the 'x' in the masthead, and
logging out by clicking 'Log out' in the masthead, work the same before
and after the patch.

Sponsored-by: Catalyst IT
Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>